There are four bosses scattered in the four corners of the field and they are all mandatory to complete the mission. Tarantulas have attacks that deplete HP and AP, so if you rely on command materia in your normal strategy, you have to make sure you avoid the Anti-AP Field by running out of range (there's plenty of room). Costly Punch is really valuable in these battles, so take advantage of the ability. Otherwise, look for opportunities to use Hell magic or critical hits. If you’re running low on MP or AP, either use Ethers and Somas or avoid using AP or MP in the random battles. The random battles can make this a lengthy mission, but there's no need to rush through. Take the EXP and gil and heal in between as necessary.